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Published: Mar 21, 2015 License: MIT



wercker status

A flexible image server written in go, using chained image filters.

Two backend are available : a pure Go backend, and a ImageMagick/GraphicsMagick backend.

Status: alpha

Building the server (pure Go):

go build -o impressionist server/pure/server.go

Building the server (ImageMagick):

go build -o impressionist server/magick/server.go

On ubuntu, you will need to install libmagickcore-dev and libgif-dev

Building the server (GraphicsMagick):

go build -tags gm -o impressionist server/magick/server.go

Running the server :

impressionist -cfg impressionist.json [-debug]


Example configuration file :

	"http": {
		"port": 80,
		"root": "/impressionist",
		"timeout": "30s",
		"workers": 10
	"filters": [
			"name": "small",
			"definition": "s,100x0"
	"image": {
		"quality": 90
	"storages": [
			"name": "local",
			"type": "local",
			"path": "/tmp"
	"caches": {
		"source": 100


One type of storage is currently supported : local.

URL structure


  • :filter is a comma separated list of filters, as explained below,
  • :format see below,
  • :storage is the name of the storage, as defined in the configuration,
  • :path is the path of the image, relative to the storage root.


  • crop : c,[x]x[y]-[w]x[h]
  • resize : s,[w]x[h][+|-]
  • flip : f,[h|v] h = horizontal, v = vertical
  • rotate: r,[90|180|270]
  • grayscale: gs
  • blur: b,s s = float (e.g: 1.0)

Note on resize :

  • s,500x100 resizes exactly to 500x100
  • s,500x0 and s,0x100 resize to a width of 500px (height of 100px), keeping the ratio
  • s,500x100+ resizes to the maximum size within the specified dimensions, keeping the ratio
  • s,500x100- resizes to the minimum dimension above the specified dimensions, keeping the ratio, and crops to 500x100

Predefined filters can be configured.

With the following configuration :

	"filters": [
			"name": "small",
			"definition": "s,100x0"

/impressionist/small/:format/:storage/:path is translated to /impressionist/s,100x0/:format/:storage/:path.

Filters supported by each backend :

filter pure Go magick
crop yes yes
resize yes yes
flip yes yes
rotate yes no
grayscale yes yes
blur yes no


  • PNG: png,
  • JPEG: jpeg[,quality] - if not specified, quality is set to the default value (image.quality config variable, or 75 if not configured).


A in-memory cache is available. It stores source files to avoid accessing filesystem for each access.

By default, it caches 100 entries, and cache size can be changed by setting caches.source. A -1 value disables the cache.


Image manipulation is delegated to a pool of workers. If a request is not finished within the configured timeout (http.timeout, by default, 30s), the request returns a 503 error.


This is a first draft, everything is still pretty rough

  • Tests, tests & tests
  • Clean stuff
  • Tuning
  • Add more storage types (eg: AWS, http)
  • Support environment variables for configuration tokens
  • Add more filters
  • Cache resized images
  • Secure URL using hashes
  • Token support
  • Optimize images using pngcrush/JPEGOptim/...
  • Format negociation & webp
  • Graceful restart & configuration reloading
  • Metrics




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